Problems in General Physics → Electrodynamics → Electric Capacitance. Energy of an Electric Field 
3.101. Find the capacitance of an isolated ballshaped conductor of radius R_{1} surrounded by an adjacent concentric layer of dielectric with permittivity ε and outside radius R_{2}.
3.102. Two parallelplate air capacitors, each of capacitance C, were connected in series to a battery with emf ξ. Then one of the capacitors was filled up with uniform dielectric with permittivity ε. How many times did the electric field strength in that capacitor decrease? What amount of charge flows through the battery?
3.103. The space between the plates of a parallelplate capacitor is filled consecutively with two dielectric layers 1 and 2 having the thicknesses d_{1} and d_{2} and the permittivities ε_{1} and ε_{2} respectively. The area of each plate is equal to S. Find:
3.104. The gap between the plates of a parallelplate capacitor is filled with isotropic dielectric whose permittivity ε varies linearly from ε_{1} to ε_{2} (ε_{2} > ε_{1}) in the direction perpendicular to the plates. The area of each plate equals S, the separation between the plates is equal to d. Find:
3.105. Find the capacitance of a spherical capacitor whose electrodes have radii R_{1} and R_{2} > R_{1} and which is filled with isotropic dielectric whose permittivity varies as ε = a/r, where a is a constant, and r is the distance from the centre of the capacitor.
3.108. Two long straight wires with equal crosssectional radii a are located parallel to each other in air. The distance between their axes equals b. Find the mutual capacitance of the wires per unit length under the condition b >> a.
3.109. Along straight wire is located parallel to an infinite conducting plate. The wire crosssectional radius is equal to a, the distance between the axis of the wire and the plane equals b. Find the mutual capacitance of this system per unit length of the wire under the condition a << b.
3.110. Find the capacitance of a system of two identical metal balls of radius a if the distance between their centres is equal to b, with b >> a. The system is located in a uniform dielectric with permittivity ε.
3.111. Determine the capacitance of a system consisting of a metal ball of radius a and an infinite conducting plane separated from the centre of the ball by the distance l if l >> a.
3.112. Find the capacitance of a system of identical capacitors between points A and B shown in
3.114. A capacitor of capacitance C_{1} = 1.0 μF withstands the maximum voltage V_{1} = 6.0 kV while a capacitor of capacitance C_{2} = 2.0 μF, the maximum voltage V_{2} = 4.0 kV. What voltage will the system of these two capacitors withstand if they are connected in series?
3.115. Find the potential difference between points A and B of the system shown in Fig. 3.19 if the emf is equal to ξ = 110 V and the capacitance ratio C_{2}/C_{1} = η = 2.0.
3.116. Find the capacitance of an infinite circuit formed by the repetition of the same link consisting of two identical capacitors, each with capacitance C (Fig. 3.20).
3.117. A circuit has a section AB shown in Fig. 3.21. The emf of the source equals ξ = 10 V, the capacitor capacitances are equal to C_{1} = 1.0 μF and C_{2} = 2.0 μF, and the potential difference φ_{A}  φ_{B} = 5.0 V. Find the voltage across each capacitor.
3.118. In a circuit shown in Fig. 3.22 find the potential difference between the left and right plates of each capacitor.
3.120. Determine the potential difference φ_{A}  φ_{B} between points A and B of the circuit shown in Fig. 3.23. Under what condition is it equal to zero?
3.121. A capacitor of capacitance C_{1} = 1.0 μF charged up to a voltage V = 110 V is connected in parallel to the terminals of a circuit consisting of two uncharged capacitors connected in series and possessing the capacitances C_{2} = 2.0 μF and C_{3} = 3.0 μF. What charge will flow through the connecting wires?
3.122. What charges will flow after the shorting of the switch Sw in the circuit illustrated in Fig. 3.24 through sections 1 and 2 in the directions indicated by the arrows?
3.123. In the circuit shown in Fig. 3.25 the emf of each battery is equal to ξ = 60 V, and the capacitor capacitances are equal to C_{1} = 2.0 μF and C_{2} = 3.0 μF. Find the charges which will flow after the shorting of the switch Sw through sections 1, 2 and 3 in the directions indicated by the arrows.
3.124. Find the potential difference φ_{A}  φ_{B} between points A and B of the circuit shown in Fig. 3.26.
3.127. Determine the interaction energy of the point charges located at the corners of a square with the side a in the circuits shown in Fig. 3.29.
3.129. A point charge q is located at a distance l from an infinite conducting plane. Find the interaction energy of that charge with chose induced on the plane.
3.131. A capacitor of capacitance C_{1} = 1.0 μF carrying initially a voltage V = 300 V is connected in parallel with an uncharged capacitor of capacitance C_{2} = 2.0 μF. Find the increment of the electric energy of this system by the moment equilibrium is reached. Explain the result obtained.
3.132. What amount of heat will be generated in the circuit shown in Fig. 3.30 after the switch Sw is shifted from position 1 to position 2?
3.134. A system consists of two thin concentric metal shells of radii R_{1} and R_{2} with corresponding charges q_{1} and q_{2}. Find the selfenergy values W_{1} and W_{2} of each shell, the interaction energy of the shells W_{12}, and the total electric energy of the system.
3.135. A charge q is distributed uniformly over the volume of a ball of radius R. Assuming the permittivity to be equal to unity, find:
3.136. A point charge q = 3.0 μC is located at the centre of a spherical layer of uniform isotropic dielectric with permittivity ε = 3.0. The inside radius of the layer is equal to a = 250 mm, the outside radius is b = 500 mm. Find the electrostatic energy inside the dielectric layer.
3.137. A spherical shell of radius R_{1} with uniform charge q is expanded to a radius R_{2}. Find the work performed by the electric forces in this process.
3.138. A spherical shell of radius R_{1} with a uniform charge q has a point charge q_{0} at its centre. Find the work performed by the electric forces during the shell expansion from radius R_{1} to radius R_{2}.
3.139. A spherical shell is uniformly charged with the surface density σ. Using the energy conservation law, find the magnitude of the electric force acting on a unit area of the shell.
3.141. Each plate of a parallelplate air capacitor has an area S. What amount of work has to be performed to slowly increase the distance between the plates from x_{1} to x_{2} if
3.142. Inside a parallelplate capacitor there is a plate parallel to the outer plates, whose thickness is equal to η = 0.60 of the gap width. When the plate is absent the capacitor capacitance equals C = 20 nF. First, the capacitor was connected in parallel to a constant voltage source producing V = 200 V, then it was disconnected from it, after which the plate was slowly removed from the gap. Find the work performed during the removal, if the plate is
3.143. A parallelplate capacitor was lowered into water in a horizontal position, with water filling up the gap between the plates d = 1.0 mm wide. Then a constant voltage V = 500 V was applied to the capacitor. Find the water pressure increment in the gap.
